Recently there has been a much increased interest in smoke detectors. The devices are typically mounted on a ceiling and produce a warning tone in response to the detection of smoke or products of combustion in the air. Such smoke detectors are available using different techniques for performing the actual detection of the presence of smoke, including ionization chambers and photoelectric detection of the obscuration or scattering of light caused by the presence of smoke.
One of the important considerations in designing any type of smoke detector is reducing the occurrence of false alarms. In order to reduce false alarms, many smoke detection circuits periodically sample or strobe the particular smoke detection apparatus being used. These circuits require that the smoke detection circuitry produce an alarm signal indicating the presence of smoke at its output for a predetermined number of consecutive samples before an alarm indication is provided to minimize false alarms caused by noise transients.
In many smoke detectors the time base used for strobing the smoke detection circuitry is derived from the 60 Hz A.C. line voltage. Such a technique has been found to be susceptible to false alarms in response to noise occurring synchronously with the 60 Hz line voltage. Syncrhonous noise may be caused, for example, by electronic regulators found in light dimmers, fans, etc. Many other common electrical appliances also produce noise which is synchronous with the line voltage. The voltages produced by the smoke detection elements are generally very small; and electromagnetic radiation produced by such synchronous noise sources may be picked up by the smoke detector circuitry to cause a false alarm. Such interference may also be capacitively or inductively coupled into the smoke detector circuitry.